Assessing the progress of internal instability of fill dam material in South Korea through long-term seepage tests

Several studies investigating the internal instability of well-graded soils have suggested that well-graded soils show characteristics and developments different from those of gap-graded soils. Moreover, the cause and progression of long-term internal instability in well-graded soils have not yet been established. This study aimed to address this knowledge gap by conducting long-term seepage tests to analyze the progress and cause of the internal instability of well-graded soils generally utilized in fill dams in South Korea. The test results revealed that well-graded soils in internally stable conditions exhibited clogging due to fine particles, reducing overall permeability. A similar phenomenon was identified under internally unstable conditions. The subsequent unclogging process resulted in a rapid increase in permeability and erosion rate with the collapse of the soil structure. Consequently, the internal instabilities of well-graded soils progress through suffosion. Based on the test results, the mechanism of internal instability and its progression in well-graded soils were proposed.